The present invention relates to process variable transmitters used in process control and monitoring systems. More specifically, the present invention relates to monitoring EMF voltage across a sensor.
Process variable transmitters are used to measure process parameters in a process control or monitoring system. Microprocessor-based transmitters often include a sensor, an analog-to-digital (A/D) converter for converting an output from the sensor into a digital form, a microprocessor for compensating the digitized output, and an output circuit for transmitting the compensated output. Currently, this transmission is normally done over a process control loop, such as a 4-20 milliamp control loop, or wirelessly.
One exemplary parameter that is measured by such a system is temperature. Temperature is sensed by measuring the resistance of a restive temperature device (RTD), which is also sometimes called a platinum resistance thermometer (or PRT), or the voltage output from a thermocouple. Of course, these types of temperature sensors are only exemplary and others can be used as well. Similarly, temperature is only one exemplary process variable and a wide variety of other process control parameters can be measured as well, including, for example, pH, pressure, flow, etc. Therefore, while the present discussion proceeds with respect to a temperature sensor, it will be appreciated that the discussion could just as easily proceed with respect to sensing of other parameters.
There are a number of connection points between a temperature sensor and a measurement transmitter, that can fail or become degraded. When the connection points or measurement lines have elevated levels of resistance, small currents can be induced on these connection points or lines that impact the sensor measurement accuracy. The sensor connection points and measurement lines can become degraded, and thus exhibit these elevated levels of resistance, due to wire fraying, corrosion, or the connections can just become loose. In any of these cases, it is possible that small voltages across the temperature sensor or in the measurement loop can begin to form, and can be sensitive to temperature changes. These voltages can create measurement inaccuracies.
As one specific example, a resistive temperature detector (or RTD) ohmic measurement is generated by using up to six individual voltage points on a ratiometric calculation. All of these measurements take approximately 60 milliseconds to collect. In a typical equation for an RTD calculation, one term that can be important, and that feeds into the RTD calculation, provides significant levels of accuracy in the final output of the transmitter. This term is the residual voltage that preexists the measurement on the measurement lines, and is referred to as Vemf.
In order to obtain the value of Vemf, up to two 60 millisecond voltage measurements are taken per sensor, when no excitation current is induced across the RTD. These measurements represent thermal voltages that can be induced on the sensor wires due to fraying, corrosion, or loose connections, among other things. These measurements can be subtracted in software from the voltage drop measured across the RTD when the excitation current is present. U.S. Pat. No. 6,356,191 is directed to this process, and the process works quite well.
However, it takes time to collect the measurements for Vemf. For instance, in one conventional system, the time to collect the Vemf measurements is approximately 120 milliseconds per sensor. This can negatively affect the update rate on a temperature transmitter.